The main rotor of a helicopter, which produces lift necessary for flight, also produces a counteracting torque force on the fuselage of the helicopter. The helicopter's tail rotor, located to the rear of the main rotor, is used to counteract this torque and to control the yaw of the helicopter. Tail rotors are typically mounted on a horizontal axis perpendicular to the direction of flight of the rotorcraft. The blades of a tail rotor typically change pitch to control thrust direction and intensity.
Traditional tail rotors have several drawbacks. First, because the tail rotor blades are large and openly exposed, they are susceptible to striking people or other objects while in operation. Second, the noise produced by traditional tail rotors can be unacceptably high, creating flyover acoustic problems. Such noise may be undesirable in a low noise environment or during clandestine activities. Traditional tail rotors may also create undesirable profile drag during flight, and/or cause thrust reduction during sideward flight.
Attempts have been made to use cross-flow fans to control helicopter yaw. It has been found, however, that cross-flow fans systems require redundancy of parts and detrimentally complex gearing and clutching schemes to provide thrust in more than a single direction. For example, rotorcrafts that employ cross-flow fans with unidirectional thrust functionality may require two or more cross-flow fans pointed in opposite directions to achieve thrust in both the pro-torque and anti-torque directions. Accordingly, the need has arisen for improved anti-torque systems for use on rotorcraft that have a reduced noise signature, a reduced impact risk and a reduced profile drag while also providing thrust in more than one direction.